Spinal Column - Anatomy & Physiology
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The trunk consists of three segments; the thorax, the abdomen, and the pelvis, each of which is bounded by body wall and contains a cavity. The thoracic cavity lies cranial to the diaphragm, whereas the abdominal cavity lies caudal. The pelvic cavity is defined by the borders of the bony pelvis and communicates with the abdominal cavity. Dorsally, the roof of all three cavities is formed by the spinal column and associated muscles. Vertebrae develop segmentally from somitic sclerotomes, whereas muscles develop from somitic myotomes. Within each myotome is a single nerve leaving the central neural tube. Motor innervation in the adult is therefore segmental. The ventral and lateral body walls are initially formed by somatopleure (ectoderm and lateral plate mesoderm), but is later invaded by somitic cells migrating ventrally. These differentiate to form ribs and sternum and associated muscles.
Divisions and Landmarks
The common pattern of canine landmarks, according to Dyce, Sack, et al, can be identified as:
Vertebrae and Joints
Vertebrae consist of a body, which encloses the vertebral foramen (through which the spinal cord and meninges run), a spinous process, and a transverse process, as well as articular processes by which they join together. The form of the spinous process varies with respect to species and region.
The first two cervical vertebrae are known as the atlas and the axis respectively, and are modified to allow movement of the head. The atlas has no conventional body, instead it is composed of two lateral masses joined by dorsal and ventral arches. The atlas and axis are fused in embryonic life. The wing of the atlas is the transverse process of this vertebra and allows the spinal column to articulate with the skull, by providing a resting place for the occipital condyles. The axis is the longest vertebra. The nuchal ligament connects the spinous process of the axis to the spinous process of the first thoracic vertebra (T1). The last (C7) cervical vertebra has a taller spinous process than those preceding it, and articulates with the first pair of ribs.
Thoracic vertebrae articulate with the ribs. They are distinguished by short bodies with flattened extremities, costal facets, short transverse processes and prominent spinous processes. They reach a maximum height, a few vertebrae behind the cervicothoracic junction (constituting the withers of the horse) and then decline. The orientation of spinous processes shifts from caudo- to craniodorsal.
The lumbar vertebrae are longer and more uniform in shape than the thoracic vertebrae. They are also shorter in height, with long, flattened transverse processes that project laterally.
The sacrum is a single bone formed by the fusion of several vertebrae that articulates with the pelvic girdle. It allows the thrust of the hindlimbs to be transmitted to the trunk. The sacrum narrows caudally and is curved to present a concave surface to the pelvic cavity.
The number of caudal vertebrae varies greatly even within species. There is a progressive simplification of their form.
Joints of the Spinal Column
There are two types of joints:
Provides direct connections between vertebral bodies. The bodies of adjacent vertebrae are connected by thick, flexible intervertebral discs, consisting of two parts:
- Nucleus pulposus: slightly eccentric, notochord derivative, contained under pressure and prone to escape.
- Annulus fibrosus: encircling bundles of fibrous tissue that pass obliquely from one vertebra to another, with changing orientation.
Found between facets on vertebral arches. They are modified in the regions of the head and pelvis.
Joints of the atlas
- Atlanto-occipital joint - Between the condyles of the skull and corresponding cavities of the atlas. It functions as a ginglymus, movement is restricted to flexion/extension in the sagittal plane (eg nodding).
- Atlantoaxial joint - The ventral arch of atlas and the body of the axis face into a single synovial cavity with limited areas of contact. Movement is rotational about a longitudinal axis (eg. head shaking).
The details of the spinal cord are found on spinal cord page.
Hypaxial and Epaxial Muscles
The epaxial muscles are extensors of the vertebral column. They are found dorsal to the line of the transverse processes of the vertebrae and are arranged in three parallel columns.
1. Lateral column
- Iliocostalis arises from the ilium and transverse processes of the lumbar vertebrae to insert on cranial lumbar vertebrae and ribs, spanning about 4 vertebrae.
2. Middle column
- Longissimus is the strongest, extending from the ilium and sacrum to the head and neck.
3. Medial column
- Transversospinalis is the most complex, lying between the medial vertebral arches and the spinous processes.
Innervation comes from the dorsal branches of the spinal nerves. They are rarely of clinical importance.
The hypaxial muscles are flexors of the neck and tail.
- From the cranial thoracic region to the atlas, covering the ventral vertebral bodies
Rectus capitis ventralis
- From the atlas to ventral skull
- From the midcervical vertebrae to the skull
- From the caudal cervical vertebrae to first few ribs, which they stabilize on inspiration
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